I've got a problem with cooperating OpenBSD CARP and CISCO router.
Firstly, there are several cases to make a failover and a load balancing.
Let me introduce one of them. General network concept depicts below "diagram".

2nd (not working). I mean CISCO router cannot ping the 10.0.0.9 IP address and back, where above configuration works correctly and the IP address is pingable. In the other words, one of the host (PC-Windows) pings correctly 10.0.0.9 at both cases.

I've got a problem with cooperating OpenBSD CARP and CISCO router.
Firstly, there are several cases to make a failover and a load balancing.

Welcome!

I don't have complete answers to your questions, but although you have provided a fair amount of information upfront, you have not disclosed what may be the most important piece of information needed -- the version of OpenBSD used. At a minimum, providing the output of the following would help shed light:

$sysctl kern.version

However, providing the entire output of dmesg(8) would be better, as this provides the kernel's complete interpretation of the hardware.

In reading the project's official misc@ mailing list, there have been sporadic issues with carp(4) found in 5.3-release which have already been addressed in -current. If you are unfamiliar with OpenBSD's flavors, see Section 5.1 of the official FAQ. If you are running anything older than a recent snapshot of -current, I would encourage you to retest with something more up-to-date.

A subset of misc@ discussions on carp(4) can be found at the following:

As for your questions on load balancing using carp(4), I have not ferreted out all the issues associated with the information provided, but I question whether you should be looking at relayd(8) instead.

I apologize that I don't have a more complete answer to your questions. This is a relatively small community of OpenBSD users which is independent of the project proper. We don't have all answers, but we do try to further focus questions which are posed.

Other members here may provide further perspective at their convenience.

I don't claim to be an expert at carp(4) issues, but in my reading of misc@ through the years, mixing multiple versions of OpenBSD in a CARP configuration is not the safest bet to make. Again, I highly recommend using a recent snapshot of -current across all nodes.

I recommend you reach out to the larger OpenBSD community, via the misc@ mailing list, and provide a similar problem report there. As ocicat noted, there are only a few of us here who answer OpenBSD questions, and only a small subset will have any CARP experience. (I've only ever played with it in a lab, and that was years ago.)

Above, ocicat recommended you use -current, rather than 5.3, as CARP has undergone changes since 5.3-release. You are likely going to receive similar recommendations from respondents on misc@. Installing a -current snapshot is no more difficult than installing 5.3-release.

Installing -current is easy, but I don't recommend upgrading from 5.3 to -current at this time if you're a new user. In mid-August, internal clock values were increased to 64 bits (on all architectures), and an upgrade through this requires significant manual steps.

I'm confused by your opening statments, because your ASCII "graphic" is indecipherable to me, and because there are at least two typos in the supporting text, making it difficult for me to understand your configuration. In particular, I notices that your CIDR is a /29, not a /30, according to your netmask. That's a good thing, because a /30 can only use two IP addresses. The /29 can address six.

It is not clear that you understand what is -current. This information was provided to you in my first response to you from Section 5.1 of the project's official FAQ. -current is the head of the CVS repository for all of OpenBSD's source code. Even though OpenBSD 5.4 is to be officially released 1 November 2013, the source code was frozen for this version back in August in order to allow CD's be to manufactured & ready for distribution at the beginning of November. Development on OpenBSD continues even in spite of the projected 5.4 release date, & all development continues to be checked into CVS. The very latest source for OpenBSD, which right now is one month past what will available as 5.4-release, can be obtained now as 5.4-current at any of the OpenBSD FTP mirror sites.

If you still unsure, study the indicated FAQ section first, then post any follow-up questions here.